1. Field of the Invention
The present invention relates to a rotary body with a high parting ability, a method and a device for fixing a toner image on a recording medium by using the rotary body, and an image forming apparatus including the fixing device.
2. Description of the Background Art
Various methods and devices have been proposed in the past for fixation to be executed in an electrophotographic image forming apparatus. Among them, a fixing method that causes a rotary body, accommodating a heat source therein, and a pressing body to nip a recording medium carrying a toner image thereon is predominant today. The rotary body for such a fixing method has a surface formed of a material containing fluorocarbon resin or similar parting agent that promotes parting of toner. The recording medium is passed through a nip between the rotary body and the pressing body with the toner image contacting the surface of the rotary body. Thus, the surface of the rotary body, generally referred to as a heat roller or a fixing roller, and the toner image carried on the recording medium contact each other under pressure, so that extremely high thermal efficiency is achievable at the time of fixation. The fixing method can therefore rapidly fix the toner image on the recording medium and is desirably applicable to a high-speed electrophotographic image forming apparatus.
However, the fixing method stated above has some problems left unsolved, as will be described hereinafter. The surface layer of the rotary body is rubbed by the recording sheet under pressure and therefore wears. Further, a peeler configured to prevent the recording medium from wrapping round the rotary body is positioned in the vicinity of the rotary body. If the peeler strongly contacts the surface of the rotary body, then it is apt to peel off the surface layer. This is particularly true with fluorocarbon resin because fluorocarbon resin has a parting ability and is therefore apt to fail to strongly adhere to the under layer. Although a primer layer may be formed between fluorocarbon resin and the under layer, as proposed in the past, it is likely that fluorocarbon resin and primer layer are peeled off by an extraneous force when implemented as thin films stacked one upon the other.
To solve the problems stated above, Japanese Patent Laid-Open Publication No. 2000-298411 proposes a roller including a heat-resistant cover layer containing fluorocarbon resin and baked at 300° C. or above. On the other hand, Japanese Patent No. 3,261,166 discloses a fixing device using a roller that includes a layer containing poly-2,2′-(m-phenylene) -5,5′-bibenzoimidazole (PBI) and fluorine-based high/low molecular weight fluorine compound. In any case, a mixture of such a material and heat-resistant resin is coated on a roller surface in the form of a film.
A problem with the above mixture is that the surface structure of the resulting roller is apt to be irregular, depending on production conditions, and effects the service life of the roller. Further, Laid-Open Publication No. 2000-298411 mentioned above expects the fluorocarbon resin layer to rise to the surface of the heat-resistant resin layer. However, if the fluorocarbon resin layer is thin and has its surface scratched by one cause or another, then a polyimide layer, constituting the heat-resistant resin layer and poor in parting ability, is apt to appear on the surface and form cores on which toner deposits, bringing about a jam.